The present invention relates to a new method of constructing and interrogating electrochemical cells, especially those having lithium thionyl chloride (Li/SOCl.sub.2) chemistry, that permits rapid estimation of remaining discharge capacity. A preferred embodiment of the present invention employs cells with specially modified anode structures and a method for testing the state of charge and remaining life of these cells, as well as depassivating these cells, prior to their intended use or re-use. The test method can be performed using an inexpensive DC circuit and voltmeter at ambient conditions anytime prior to cell use or re-use.
Determining the state of charge and remaining life of batteries is important particularly where the batteries are intended to be re-used for additional applications prior to disposal. For example, a special class of high performance batteries are presently used in the oilfield service markets (i.e., markets focusing on the subterranean search for hydrocarbons). Battery re-use is desired to spread the operational cost of the specialized battery over a number of field service jobs. Since these batteries must work in tools that operate from ambient to high temperatures in confining environments, they must have high volumetric energy density and function over a wide temperature range. Lithium thionyl chloride (LTC) chemistry is most frequently used to meet these stringent conditions. However, the remaining life for this type of LTC battery is very difficult to establish because most measurable parameters (voltage, internal resistance, etc.) do not vary appreciably with depth of cell discharge until very near the cell's end of life. (See D. Linden, Handbook of Batteries, 2nd Ed., McGraw-Hill, New York, (1995) at section 14.28).
A typical LTC oilfield services battery is often designed with a useful life of up to 1700 hours (depending on the tool loads applied), is used in applications where the temperature is typically less than 130.degree. C. and the duration of the application may be than 100 hours. Therefore, if new batteries were placed in a tool, and the tool was used for one application, at the end of that application, the batteries would have sufficient life for use in one or more additional applications. However, at present, many batteries are disposed of after they have been consumed only partially and a new battery is used simply because the tool operator wants to make sure that there is sufficient life in the battery for the next operation. Such single usage is undesirable for a variety of reasons. Premature disposal of a battery wastes the remainder of the useful life of the battery, wastes battery chemistry, and creates added battery disposal costs regarding, e.g., the active unspent ingredients, e.g., lithium and thionyl chloride. As an alternative to disposing of a battery after its first use, some tool operators have attempted to maintain a log that measures and documents the battery's elapsed service life or conversely, amp-hours. However, automatic measurement of elapsed amp-hours is difficult to implement because it requires added electronic circuitry (i.e., a means for providing an on board processor capable of recording amp-hours or time) in either the battery or the tool. If the added circuitry is housed in the tool itself, then each battery needs to remain in the tool until the end of the battery's, life, otherwise, the tool's circuitry will lose track of that particular battery's elapsed amp-hours. If the added circuitry is housed in the battery itself, then each battery becomes more complicated thereby increasing expense and decreasing reliability for what is ultimately a disposable product. Neither of these options are practical in oilfield-service use where there is considerable pressure to keep disposable batteries and re-usable tools as inexpensive and simple to use as possible. Manually monitoring and maintaining a log of the number of amp-hours used in an oilfield services battery is time consuming, creates additional bookkeeping burdens, and may not consistently be carried out for each battery thereby leading to a great deal of uncertainty about a given battery cell's potential life span for use in another application. Another method of measuring battery life includes the measurement of the battery pack's DC or AC voltage; however, no clear correlation of signal to remaining capacity is obtainable, and in addition, complicated electronics are required.
Certain electrochemical cells have been described for achieving end-of-life indication during electrochemical discharge. Electrochemical cells for use in implantable medical devices have been described for generating a step change in output voltage during cell operation/discharge and prior to full cell discharge to enable the timely replacement of the cell under non-critical circumstances. Battery powered implantable medical devices require some means of end-of-life indication for the battery so that physicians have sufficient notice, or elective replacement indication, to complete replacement of the medical device and/or power source prior to system failure brought about by battery power depletion. For example, U.S. Pat. No. 4,293,622 to Marincic, et al., U.S. Pat. No. 4,563,401 to Kane, et al., and U.S. Pat. No. 5,569,553 to Smesko, et al., (all of which are incorporated herein by reference) describe electrochemical cells, particularly suitable for use in surgically implanted devices, which exhibit a step change in output voltage during operation/discharge of the cell sufficiently prior to full cell discharge to enable the timely replacement of the cell under non-critical circumstances. The cells of Marincic, Kane, and Smesko could thereby be casually replaced only when necessary, and before the depth of discharge has become critical.
The Marincic and Kane patents include lithium/thionyl chloride electrochemical cells characterized by an operating voltage which is essentially independent of the degree of cell discharge. Such cells are known to employ an electrolyte which includes an oxyhalide depolarizer together with a Lewis acid and Lewis base solute, a cathode structure formed from a finely ground metallic powder capable of functioning as a catalyst for oxyhalide reduction, and an alkali metal anode. The oxyhalide depolarizer may, for example, be thionyl chloride, sulfuryl chloride or phosphoroxy chloride. A typical Lewis acid and Lewis base is aluminum chloride and lithium chloride. The cathode structure may comprise carbon, platinum, or palladium. The alkali anode material may illustratively be lithium, calcium, sodium or potassium. Other materials may be suitable. As recognized by the Marincic and Kane patents, although such cells have greatly simplified the design of most electronic devices by substantially eliminating the need to consider voltage variations during essentially the entire life of the cell, this "flat voltage" characteristic has been a source of concern to designers of certain devices, notably pacemakers, infusion pumps, and other surgically implanted devices. Marincic and Kane note that the reason for concern is, of course, the inability to voltaicly determine the actual state of cell discharge and to predict the end of life prior to full cell discharge to enable the timely replacement of the cell under non-critical circumstances.
The Marincic patent teaches the use of an electrolyte-limited cell, i.e., a cell having a ratio of active components which result in the exhaustion of the thionyl chloride electrolyte prior to the other active materials. Electrolyte-limited cells show a step in output voltage when the cell is designed to avoid polarization; accordingly, attention is given to ensure that the cathode structure is not rate-limited below the anticipated discharge rate of the cell. The manifestation of the step is a product of the stoichiometry of the cell. The Kane patent discloses a cell construction offering an operating voltage which is essentially independent of the degree of cell discharge except for a desired number of step changes. Each of the step changes is, by cell design, of a desired magnitude and occurs at a pre-selected degree of cell discharge. The electrochemical cell of Kane comprises anode-functioning means, cathode functioning means, and electrolyte functioning means formed of respective materials which provide an electrochemical system characterized by an operating voltage essentially independent of the degree of cell discharge. The anode-functioning means includes a plurality of electrochemically active surface members arranged to become non-active at mutually different degrees of system discharge. In an embodiment described in Kane, there are two anode members having cylindrical shapes of non-uniform length to provide a greater thickness where the anode members are coextensive. A cylindrical cathode is provided radially inside the anodes. As each surface member becomes non-active, the remaining surface members must support the current capability of the cell depleted, increasing the internal cell resistance and consequently reducing the cell's operating voltage. As disclosed in Kane, in operation, the discharge rate over the entire surface of the anode material is the same. Accordingly, the thinner portion of the anode structure will become depleted first. The resultant lesser surface area of remaining anode material must support an overall current of the same magnitude. Since the total amount of anode surface area has been reduced, so has the current capability of the partially discharged cell. The consequential reduced output voltage level is a function of the discharge rate of the cell, and the amount of anode surface area remaining. It is thus controllable by adjusting the ratio of the surface area of the thinner anode layer to that of the thicker anode layer. By adjusting the actual layer thicknesses of the anode members of the Kane cell, the point or points at which the step change is manifested can be adjusted. The Smesko, et al. patent discloses, e.g., spiral wrapped or jelly roll cell design used to create a step change in the cell's voltage during operation. In each of the Kane, Smesko, and Marincic cells, the onset of reduced voltage output acts as an end-of-life indicator.
The Kane, Marincic and Smesko patents discussed above disclose electrochemical cell configurations designed to generate a step-wise, detectable reduced energy output so that the cell's end-of-life can be detected in situ during the cell operation to avoid, e.g., unnecessary replacement of a battery in a surgically implanted device. It would appear that these surgically implanted devices are placed into the body with the goal of detecting the battery end of life before a critical time while minimizing the need to perform additional surgical procedures to check on the battery life or to prematurely change a battery. The batteries used in surgically implanted devices therefore appear to be designed for a single use, and, as a consequence, must be monitored in situ to maximize the battery's useful life without resorting to additional surgical procedures to accomplish such monitoring. However, neither Kane, Marincic, nor Smesko describe how to actually monitor when the cell's end-of-life is near. It would be reasonable to believe that the running voltage of a cell contained in a surgically implanted device is monitored during the operation of the cell.
During application of a tool, such as an oilfield services tool, the use a battery cell construction such as that described in Kane, Marincic or Smesko would be possible. However, the running voltage of the tool employing such cell can not be monitored during use, or would require that modifications be made to the tool to accommodate such monitoring thereby increasing the complexity and cost of the tool while potentially lowering the overall reliability of the tool and also precluding its implementation in preexisting tools. Further, the teachings of Kane, Marincic and Smesko do not address whether such end of life detection mechanisms would operate properly at temperatures higher than body temperature. For example, in the oilfield services area, a cell may be subjected to a range of operational temperatures during one use or during different uses. Therefore, a need exists for determining the state of charge or end of life of a battery that is potentially subjected to varying temperature conditions, including those ranging between ambient and 130.degree. C. Also, in oilfield operations, unlike as taught in Marincic, the use of a thionyl chloride-limited cell presents a safety hazard if the liquid cathode/electrolyte, here, thionyl chloride, is consumed prior to the lithium anode. The cells used in the oilfield need to have an excess amount of cathode (i.e., excess thionyl chloride) in relationship to the lithium anode.
Additionally, if a battery is not disposed after its first use, then the battery either remains stored within the tool, or is taken out of the tool and placed in a storage area. Also, new batteries may often be stored for significant time prior to first use. Therefore, it is important to note that the general voltaic performance of LTC batteries depends greatly at ambient temperatures on the growth of LiCl surface layers on the internal electrodes which results in cell passivation. (See J. P., Gabano, Lithium Batteries, Academic, London, (1983)). Passivation greatly effects the voltaic performance of LTC batteries. The passivation effect results in a voltage delay, or the inability of the battery to hold its voltage under a given load when it is next used. At elevated temperatures such passivation effects are significantly diminished. The voltage delay becomes more pronounced with a heavier discharge load and lower discharge temperature, and is more severe in the Li/SOCl.sub.2 than it is for other lithium batteries. Once the cell discharge is started, the passivation film layer is thinned gradually, the internal resistance returns to its nominal value, and the working voltage is reached. The extent of cell passivation depends partly on the cell thermal/load history and the cell test temperature. The passivation layer or film may be removed rapidly by application of high-current pulses for a short period or, alternatively, by applying a low load for many hours. However, many tool current loads are not high enough to depassivate a passivated LTC battery. Any use of a passivated LTC battery may create the potential for a tool misrun due to battery failure. Consequently, the study of LTC batteries by electronic interrogation means, including inferring remaining life by monitoring step voltage changes, is complicated by the added internal resistance resulting from cell passivation.
As a result, it became necessary to design a low cost, reliable and simple method and apparatus for determining the state of charge and useful life of a LTC-type battery system and to eliminate any existing passivation of such batteries prior to their use or reuse. This methodology is preferably compatible with existing tools or devices using such batteries, such as oilfield service tools, and provides an alternative to a hands-on record keeping for each battery or the use of on-board battery electronics. Additionally, this methodology preferably allows for battery pack removal from the tool, is not tool dependent and does not use up battery life by requiring additional electronics.